Wet Spots as Hotspots: Moisture Responses of Nitric and Nitrous Oxide Emissions From Poorly Drained Agricultural Soils

Abstract

A classic framework for soil nitrogen (N) cycling, the hole in the pipe (HIP) model, posits a trade-off in emissions of nitric oxide (NO) and nitrous oxide (N 2 O) as a function of soil moisture. This has been incorporated into ecosystem models but not tested experimentally and remains an important uncertainty for understanding potential hotspots of reactive N emissions: poorly drained agricultural soils that experience episodically high moisture following intensive fertilization. We incubated soils at moisture ranging from 44% to 100% water-filled pore space (WFPS). Counter to HIP, we did not observe a consistent trade-off in NO and N 2 O emissions at intermediate moisture levels following fertilization, and prefertilization emissions were low. Emissions of N as N 2 O exceeded NO by 2-200-fold at all moisture levels and peaked at 73-82% WFPS. Emissions of NO declined with moisture but remained significant even under saturated conditions. Increases in nitrite and reduced iron at high moisture indicated possible NO production from chemodenitrification. Potential nitrification rates were 100-1,000-fold greater than potential denitrification. Emission factors for fertilizer N ranged from 0.05% to 0.58% (mean = 0.2%) for NO and from 0.4% to 16.9% (mean = 5.3%) for N 2 O. Our results caution the use of WFPS to predict NO:N 2 O emission ratios as often employed in ecosystem models. Subsurface N cycling may suppress emissions of NO relative to N 2 O, and N 2 O emissions can persist under saturated conditions. Elevated N 2 O emissions from in-field wet spots comprising a small landscape extent could potentially address disparities between top-down and bottom-up N 2 O budgets.Plain Language Summary Nitric oxide and nitrous oxide are environmentally harmful gases: they contribute to climate change, the loss of good ozone high in the atmosphere that protects us from damaging sunlight, and the production of bad ozone that pollutes our air. The activities of microorganisms in soils are important sources of these gases to the atmosphere, and fertilized agricultural soils have particularly high emissions. Conceptual and mathematical models are often used to predict how nitrous and nitric oxide emissions respond to environmental changes and to guide policy and management. Here we tested a key assumption of a classic conceptual model that is widely used to estimate soil nitrous and nitric oxide emissions, using a laboratory experiment with two poorly drained agricultural soils. We found that there was not a clear trade-off in the emissions of nitric versus nitrous oxide as soil moisture varied from low to high levels, in contrast to predictions. Rather, emissions of nitrous oxide were greater than expected, and production of nitrous oxide and nitric oxide persisted under very wet conditions. Poorly drained patches of intensively fertilized agricultural soils could represent important sources of nitrous oxide to the atmosphere. HALL ET AL.3589

Legacy Effects of Intercropping and Nitrogen Fertilization on Soil N Cycling, Nitrous Oxide Emissions, and the Soil Microbial Community in Tropical Maize Production

“…In our study, cumulative N 2 O emissions were correlated with WFPS regardless of the time point of soil sampling, and WFPS explained 26, 65, and 33% of N 2 O data variance at 0, 16 and 49 DAT, respectively (Table 2). Higher WFPS results in lower O 2 availability and consequently lower redox potential, creating a more favorable condition for denitrification than the other treatments (66). However, we did not identify strong evidence of anaerobic taxa, which suggest that soil conditions do not match with ideal conditions where denitrification is the main biogeochemical process driving N 2 O emission (66).…”

Iron Redox Reactions Can Drive Microtopographic Variation in Upland Soil Carbon Dioxide and Nitrous Oxide Emissions

“…The abiotic oxidation of less than 2% of the Fe(II) amendment amount could sustain this level of N 2 O production over 24 h. It is possible that rapid chemodenitrification occurred immediately after flooding the soil cores and that we did not capture this initial pulse and subsequently underestimated the contribution of chemodenitrification to N 2 O emissions. While both biotic and abiotic reactions have been shown to produce N 2 O when Fe(II) oxidation is coupled to the reduction of inorganic N compounds [27,28,30,31,51,52], it is likely that abiotic Fe(II) oxidation was the dominant process since it is significantly faster than biotic Fe(II) oxidation [7,27]. However, it is possible that biotic pathways also contribute, either by directly oxidizing Fe(II) for energy or NO 2 − detoxification, or by denitrifying other inorganic N compounds produced by abiotic Fe(II) oxidation [27,30].…”

What can we learn from N2O isotope data? – Analytics, processes and modelling

“…Although these simple parameterization methods provide the first step in constraining emission processes, recent results highlight the importance of also considering other drivers in these models, such as pH regulation of nitrification 77 and N 2 O reduction, microbial biomass and land use history, and substrate mobilization and availability 36,78 . The N 2 O emissions during “hot moments” and from “hot spots” in the environment are also increasingly recognized as playing a major role in annual and regional N 2 O budgets, but their controls are particularly challenging to understand in their full complexity, and thus difficult to model 79–82 . Using natural abundance and isotope labelling approaches to gain a mechanistic understanding of the response of N 2 O transformation pathways to the most important drivers will be key to improving models and allowing predictions of the N 2 O budget in heterogeneous environments, in particular in the context of a changing climate.…”